Evaluating Effective Lengths Of Columns In The Staggered Truss Structure

In the strong-bracing staggered truss structure, the rotating restraints which the columns suffer come from not only the members of the truss, but also the columns up and down. The axial force sometimes is so large that the restraints are negative, which hasten the columns buckling, bring the negative restraint effect. The paper regards the connected up-and-down columns as the restraining members and gains the columns' buckling equation, enough considering disturbance by the axial force on the restraining members and interaction between columns and restraint members. Later, the paper brings forward the R-factors method to calculate the columns' effective length and provides more premise applied calculating formula. At the same time, it is drew that the charts on the effective length factor, which shows that the effective length factor of columns buckling in the sway-prevented mode will be larger than 1.0. The paper also approximately considers the restraints from the members of neighboring truss, by modifying the rotating restraint stiffness of chords connecting the columns. After revising the effective length factor to consider the integer stability of the columns, the paper finally provides the whole and systemic R-factors method for the columns' effective length in the strong-bracing staggered truss structure.The columns' buckling in the weak-bracing staggered truss structure is differing from that in frame structure, because of the trusses' bracing. The paper equally changes the bracing from the trusses into lateral bracing spring on the columns' ends, to set up the model for analyzing. After considering the restraints from the members of neighboring truss, the paper gains the columns' buckling equation and brings forward the R-TK factors method to calculate the columns' effective length, providing corresponding calculating formula. It is found that the columns' buckling mode will change, when the relative bracing stiffness reaches some value. Therefore, the paper introduces the critical relative bracing stiffness into judge the buckling mode of columns and correspondingly cite the critical sway-side stiffness of the truss as a method to judge whether the staggered truss structure is a strong-bracing one or a weak-bracing one. After considering the integer stability of the columns, the paper finally provides the whole and systemic R-TK factors method to calculate the columns' effective length in the weak-bracing staggered truss structure.Through the examples, it is validated that the value by the two above methods isso premise and safe that serves completely the requests of the practical designing. With the truss floor and open floor staggered, the columns' actual effective length factor in the staggered truss structure appears to increase crossbeddedly from the bottom to top, which increase more obviously in the open floor.